1. Field of the Invention
The present invention relates to a focus monitoring method, a focus monitoring system, and a device fabricating method. More particularly, the invention relates to a focus monitoring method, a focus monitoring system, and a device fabricating method for use in generation of a pattern of a device.
2. Description of the Background Art
Increase in packing density and reduction in size of a semiconductor integrated circuit in recent years is remarkable. In association with this, a circuit pattern formed on a semiconductor substrate (hereinafter, simply called a wafer) is rapidly becoming finer.
In particular, the photolithography technique is widely recognized as a basic technique of generating a pattern. It has been therefore variously developed and modified until today. The pattern is continuously becoming finer and a demand on improvement of the resolution of the pattern is increasing.
The photolithography technique is a technique of transferring a pattern on a photo mask (original image) onto a photoresist applied on a wafer and patterning an underlayer to be etched by using the transferred photoresist.
At the time of transferring the photoresist, the photoresist is subjected to a developing process. In a photoresist of the positive type, the photoresist irradiated with light in the developing process is removed. In a photoresist of the negative type, the photoresist which is not irradiated with light is removed.
Generally, a resolution limit R(nm) in the photolithography technique using a stepper method is expressed as follows.
R=k1xc2x7xcex/(NA) 
where xcex is a wavelength (nm) of light used, NA denotes numerical aperture of a projection optical system of a lens, and k1 is a constant depending on image forming conditions and resist process.
As understood from the expression, to improve the resolution limit R, that is, to obtain a finer pattern, a method of setting each of k1 and xcex to a small value and setting NA to a large value can be considered. That is, it is sufficient to set a lower constant which depends on the resist process, shorten the wavelength, and set a larger NA.
It is however difficult from the technical point of view to improve a light source and a lens. By shortening the wavelength and setting a larger NA, a focal depth xcex4 (xcex4=k2xc2x7xcex/(NA)2) of light decreases, and a program such as deterioration in the resolution arises.
In the photolithography technique, to expose and transfer the pattern of a photo mask onto a photoresist with high resolution, the photo mask has to be exposed in a state where the photoresist is fit in the range of the depth of focus with respect to the best focus plane of the projection optical system. For this purpose, the position of the best focus plane of the projection optical system, that is, the best focus position has to be calculated by any method.
An example of a conventional focus monitor for measuring the best focus position is a phase shift focus monitor developed by Brunner of IBM corporation and sold by Benchmark Technology Co., U.S.A.
FIG. 16 is a diagram for explaining a phase shift focus monitoring method. Referring to FIG. 16, the phase shift focus monitoring method uses a phase shift mask 105. Phase shift mask 105 has a transparent substrate 105a, a shielding film 105b having a predetermined pattern, and a phase shifter 105c formed on the predetermined pattern.
Phase shift mask 105 has a pattern in which, as concretely shown in FIG. 17, narrow shield pattern 105b is provided between sufficiently thick transmitting portions 105d and 105e. Phase shifter 105c is not disposed in transmitting portion 105d but is disposed in transmitting portion 105e. 
According to the phase shift focus monitoring method, referring to FIG. 16, first, phase shift mask 105 is irradiated with light. Since phase shifter 105c is constructed to shift the phase of transmission light by about 90 degrees, in the case where light passed through transmitting portion 105e travels faster than light passed through transmitting portion 105d by an optical path difference of 1/4xcex, 5/4xcex, . . . or in the case where the light passed through transmitting portion 105e travels behind the light passed through transmitting portion 105d by an optical path difference of 3/4xcex, 7/4xcex, . . . , the light strengthens with each other. Consequently, light passed through phase shift mask 105 has an intensity distribution asymmetrical with respect to the z axis (optical axis). Light passed through phase shift mask 105 is condensed by projection lenses 119a and 119b, and an image is formed on a photoresist 121b on a semiconductor substrate 121a. 
By the phase shift focus monitor, an image is formed on photoresist 121b in a state where the intensity distribution of diffracted light is asymmetrical with respect to the z axis. With movement of a wafer 121 in the z direction, an image of a pattern on the wafer 121 therefore moves in the direction (x-y direction, that is, the lateral direction of the drawing) perpendicular to the z axis (vertical direction of the drawing). By measuring the shift amount of the image of the pattern in the x-y direction, the position in the z direction, that is, focus can be measured.
In the conventional phase shift focus monitoring method, to obtain high detection sensitivity in the z direction (ratio of the shift amount in the x-y direction to the shift amount in the z direction), isotropic illumination having a small angle (circular shape in a pupil plane), that is, illumination having a small "sgr" value has to be used. This is described by T. A. Brunner et al., xe2x80x9cSimulations and experiments with the phase shift focus monitorxe2x80x9d, SPIE Vol. 2726, pp. 236-243. FIG. 4 of this literature teaches that, when the "sgr" value is 0.3, the shift amount (focus monitor overlay error) in the x-y direction of a pattern is the largest, and the detection sensitivity in the z direction increases.
In order to obtain illumination having a small "sgr" value as described above, for example, as shown in FIG. 18, the diameter of an open portion 14d of an illumination aperture 14 has to be decreased.
However, when a device pattern is formed with illumination having the "sgr" value as small as about 0.3, coherence of light is too strong, and deformation of a two-dimensional pattern transferred onto the photoresist is remarkable. To suppress such deformation of a two-dimensional pattern, the diameter of the illumination aperture 14 used at the time of forming a device pattern is set to be larger than that of illumination aperture 14 used at the time of monitoring a focus, and the "sgr" value has to be set to, for example, 0.6 or higher. Consequently, different illumination apertures 14 have to be used for monitoring of a focus and formation of a device pattern. There is a problem such that effort and management for changing illumination aperture 14 are required.
When oxygen entered in the illumination optical system at the time of the change remains, the lens is clouded up. Consequently, the oxygen has to be purged by introducing nitrogen for long time after the change, and a problem such that the work becomes complicated occurs.
An object of the invention is to provide a focus monitoring method, a focus monitor system, and a device fabricating method with high detection sensitivity in the z direction while an illumination aperture does not have to be changed.
According to the invention, there is provided a focus monitoring method used for forming a pattern of a device, wherein a pattern of a photo mask for phase shift focus monitor, the photo mask having first and second light transmitting areas adjacent to each other while sandwiching a shielding film, is transferred onto a photosensitive member on a substrate by using modified illumination, the pattern being constructed so that a phase difference other than 180 degrees occurs between exposure light passed through the first light transmitting area and exposure light passed through the second light transmitting area.
According to the focus monitoring method of the invention, by using the modified illumination, without setting a low "sgr" value, the detection sensitivity in the z direction as high as that in the case where standard illumination is used and a low "sgr" value is set can be obtained. Since it is unnecessary to set a low "sgr" value, the illumination aperture does not have to be changed between the time of focus monitoring and the time of formation of a device pattern.
In the focus monitoring method, preferably, the modified illumination is quadrupole illumination by which a shape of a light portion in a pupil plane obtained by eliminating a portion of a cross shape from a circular shape is formed.
By using the quadrupole illumination as modified illumination, the detection sensitivity in the z direction as high as that in the case where standard illumination is used and a low "sgr" value is set can be obtained.
In the focus monitoring method, preferably, the modified illumination is zonal illumination by which a shape of a light portion in a pupil plane, that is a zonal shape sandwiched by two concentric circles is obtained.
As described above, by using, other than the quadrupole illumination, the quadrupole illumination as modified illumination, the detection sensitivity in the z direction as high as that in the case where standard illumination is used and a low "sgr" value is set can be also obtained.
In the focus monitoring method, preferably, when a reduction ratio is r, a ratio (a/R) between a sine (a) of a maximum incident angle onto the photo mask of illumination light generated by an illumination optical system and a sine (R) of a maximum incident light angle in an image formed on the substrate by a projection optical system is 0.9/r or higher.
With the configuration, a preferable detection characteristic in the z direction can be achieved.
In the focus monitoring method, preferably, a ratio (b/R) between a width (2b) of the cross-shaped portion and a diameter (2R) of a pupil is 0.30 or higher.
With the configuration, an excellent pattern can be formed with standard illumination at the time of forming a device pattern.
In the focus monitoring method, preferably, a ratio (b/R) between a diameter (2b) in a pupil plane of an inner circle in the two concentric circles and a diameter (2R) of a pupil is 0.50 or higher.
With the configuration, an excellent pattern can be formed with standard illumination at the time of forming a device pattern.
According to another aspect of the invention, there is provided a focus monitor system used for generation of a pattern of a device, including: an illumination optical system for irradiating a pattern of a photo mask for phase shift focus monitor with modified illumination, the photo mask having first and second light transmitting areas adjacent to each other while sandwiching a shielding film and being constructed so that a phase difference other than 180 degrees occurs between exposure light passed through the first light transmitting area and exposure light passed through the second light transmitting area; and a projection optical system for projecting an image of the pattern of the photo mask onto a photosensitivity member.
According to the focus monitor system of the invention, by using the modified illumination, without setting a low "sgr" value, the detection sensitivity in the z direction as high as that in the case where standard illumination is used and a low "sgr" value is set can be obtained. Since it is unnecessary to set a low "sgr" value, the illumination aperture does not have to be changed between the time of focus monitoring and the time of formation of a device pattern.
A method of fabricating a device is characterized by using any of the focus monitoring methods.
Consequently, the high detection sensitivity in the z direction is achieved and the changing of the illumination aperture becomes unnecessary, so that a very accurate pattern can be formed without requiring management and effort of a changing work.
The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.